The uptake of thymidine and uridine in Tetrahymena pyriformis was investigated using the glass membrane filter method. The uptake of both of these nucleosides at 2 μM was temperature-and energy-dependent. The optimal pH was 5.5. The uptake of both was competitively inhibited by other nucleosides. Thymidine and uridine were considered to be transported by carrier mediated systems, and these mechanisms are discussed.